In my opinion, if a baby is small and losing weight and flat out refusing f/t, just offer it live to get its weight up first before attempting to convert to f/t.

My first baby ball (who is now a year and a half old) refused both f/t and rats, so I had to work on both. I offered him live mice up until he was around 350g. Then I first worked on converting to f/t mice, and then when f/t was more or less stable, converting from f/t mice to f/t rats. Now he consistently eats f/t rats, besides fasting sometimes.